1. Field of the Invention
The present invention relates to probe pins and, more particularly, to a spring-based probe pin that allows kelvin testing.
2. Description of the Related Art
A bumped die package is a package that eliminates the need for wire bonding and molding. With a bumped die process, solder bumps are mounted to the bond pads of a wafer, which is then diced to form a large number of individual die. Once diced, an individual die is ready to be used by a customer.
Final electrical testing is performed using a test and evaluation circuit that includes a number of probe pins. In operation, the probe pins are brought into electrical contact with the solder bumps. Voltages and currents are then applied to the solder bumps via the probe pins so that the proper operation of the circuits can be verified.
In current-generation test and evaluation circuits, the probe pins are commonly implemented as pogo pins or cantilever pins. A pogo pin is typically implemented as a cylindrically-shaped structure that houses a spring, although other types of spring-based probe pins can alternately be used.
FIG. 1 shows a cross-sectional diagram that illustrates an example of a prior-art, non-cylindrical, spring-based probe pin 100. As shown in the FIG. 1 example, probe pin 100 is implemented as a single metal element that has a top end section 110, a bottom end section 112, and a C-shaped section 114 that connects sections 110 and 112 together.
FIG. 2 shows a cross-sectional diagram that illustrates an example of a prior-art electrical test. As shown in FIG. 2, a test and evaluation circuit 210 is utilized to verify the electrical operation of the circuits on a wafer 212, which has a large number of solder bumps 214 attached to the pads of wafer 212.
Test and evaluation circuit 210, in turn, includes a printed circuit board (PCB) 210A, a socket 210B that is connected to PCB 210A, and a large number of probe pins 216 that are connected to PCB 210A and socket 210B. Each probe pin 216, in turn, is implemented as a probe pin 100, and has a top end section 220 that is electrically connected to PCB 210A, and a bottom end section 222 that is connected to a solder bump 214 during a test.
In operation, when a force is applied to bring a solder bump 214A and a probe pin 216A into electrical contact, solder bump 214A and probe pin 216A approach each other along a line parallel to a line 224 that passes through the center and apex of solder bump 214A. When probe pin 216A contacts the apex of solder bump 214A, the C-shaped section of probe pin 216A functions as a spring that absorbs some of the contact force, thereby reducing any damage to solder bump 214A that may result from the contact.
When testing, solder bump-to-probe pin misalignment errors are to be avoided as a misalignment error which causes probe pin 216A to miss the apex of solder bump 214A can cause severe damage to solder bump 214A. One significant disadvantage of probe pin 216A, and pogo pins in general, is that conventional spring-based probe pins can not be used in a kelvin test. Kelvin tests are used to accurately measure the voltage across a device by minimizing the impact of contact resistance on the device.
FIG. 3 shows a block diagram that illustrates an example of a prior-art kelvin test apparatus 300, used to measure the impedance of device 310. As shown in FIG. 3, apparatus 300 includes a device 310, such as a winding, which has an input IN and an output OUT. In addition, apparatus 300 has a voltmeter 312 and a current source 314 that are connected between the input IN and the output OUT of device 310.
In operation, to measure device 310, a test current I is passed through device 310, and the resulting voltage produced across device 310 is measured. A kelvin test allows the voltage to be accurately measured because the impedance of voltmeter 312 is very high (effectively allowing the lead-line and contact resistance to be neglected). The impedance of device 310 can then be determined by dividing the voltage by the current.
Thus, to perform a kelvin test on a bumped die package, two separate contacts must be made with a solder bump: one to provide current and one to measure voltage. Since spring-based probe pins are axially aligned with the solder bumps to contact the apexes of the solder bumps, it is not possible for two conventional spring-based probe pins to make an electrical connection with a single solder bump without severely damaging the solder bump.
Cantilever pins, on the other hand, are not axially aligned with the solder bumps on the bumped die package. Instead, cantilever pins approach the solder bumps from the side. As a result, a pair of cantilever pins can be used to contact a single solder bump, thereby allowing a kelvin test to be performed.
One disadvantage of cantilever pins, however, is that the cantilever pins can significantly damage the solder bumps when the cantilever pins and solder bumps are brought into contact. Since cantilever pins approach the solder bumps from the side, it is not uncommon for a cantilever pin to first make contact with a solder bump, and then undesirably scratch across the surface of the solder bump as the full contact force is applied to make the electrical connection.
Thus, there is a need for an approach to testing bumped die packages that minimizes damage to the solder bumps while at the same time allowing a kelvin test to be performed.